The present application relates to the manufacture of quartz glass products having new or improved properties and to novel processes for making such products from porous silica preforms. One preferred embodiment of the invention involves nitrided vitreous quartz products with outstanding physical properties. Another embodiment involves the use of the quartz glass products of this invention in connection with furnaces or high-temperature equipment, particularly in the semiconductor industry. Other embodiments relate to opaque or porous silica glass products that can function as insulation or radiation heat shields. One favored embodiment of the invention involves the use of refractory dopants, such as silicon carbide, silicon nitride, silicon oxynitride or other suitable metal nitrides.
It has been known for many years that nitrides of silicon have properties different from silicon dioxide and that some of these properties might be advantageous in certain applications. Silicon nitride and silicon oxynitrides can be produced in various ways as by reaction of silicon and/or silicon dioxide with ammonia, and products of this type would have utility for some special applications.
However, as pointed out in more detail in said copending application, there are many reasons why the commercial use of such products has been very limited, why research relating to nitrided silicon products has not been extensive, and why large capital investment for research and development in this area did not appear to be justified. It is difficult and expensive to produce silicon nitride products or silicon oxynitride products. Silicon dioxide (silica) does not react readily with nitrogen, although it is possible with appropriate reaction conditions to produce oxynitrides by reacting particles of silica with anhydrous ammonia.
Prior to the present invention, the presence of significant amounts of chemically-bound nitrogen in a quartz glass used in semiconductor manufacture would have been considered highly undesirable. Nitrogen heretofore appeared to be an impurity to be avoided.
The percentage of the nitrogen impurity in a commercial quartz glass is low but is not often measured or reported because of the difficulty of ascertaining the nitrogen content with reasonable accuracy. The analytical detection problem is another good reason why the unusual properties and advantages of chemically-bound nitrogen were heretofore not understood nor appreciated in the glass industry.
For several decades vitreous silica products essentially free of crystalline silica have been used extensively because of exceptional thermal shock resistance and other advantageous physical properties. However, these products have a limited useful life when heated above 1200xc2x0 C. and other disadvantages because of limited resistance to deformation, the devitrification of the glass, and the damage resulting from the crystallographic alpha-beta inversion during heating and cooling of the devitrified glass. There has been a need for a practical solution to these problems for several decades, particularly the devitrification problem, but no simple solution was found prior to the present invention.
From the early 1960""s to about 1990, horizontal tube furnaces were mainstay equipment in the semiconductor industry for oxidation of silicon, diffusion, heat treating and various deposition processes. They are sometimes called simply xe2x80x9cdiffusion furnacesxe2x80x9d but the more correct generic term is a tube furnace.
An important part of processing equipment is the reaction chamber. The associated process tube and wafer carriers can be formed of silicon carbide or high-purity quartz glass. Silicon carbide is a superior material because it is structurally stronger, has a longer useful life, and does not break down with repeated heating and cooling; but the use of silicon carbide tubes and wafer boats has been slowed down because of cost and weight.
Quartz is highly purified glass favored for its inherent stability at high temperatures and its cleanliness. Drawbacks to this glass are its fragility and the tendency to break up and sag after extended use at temperatures above 1200xc2x0 C. The breakup or devitrification of the quartz results in small particles or flakes of the quartz tube falling onto the silica wafers. Sagging can impede the placement of the wafer holders or cassettes in and out of the tube.
Vertical tube furnaces have smaller cleanroom foot prints, are better suited to automation, and offer other advantages over horizontal furnaces. The vertical furnaces are preferred for a number of wafer treatment processes including chemical vapor deposition (CVD).
The semiconductor industry is continually striving for greater uniformity and higher process yields. In order to achieve more precision and greater efficiency, the furnaces, reactors and other high-temperature equipment need proper non-contaminating insulation and more efficient means for reducing radiation heat losses. Prior to the present invention, the attempts to meet these needs have been crude and generally unsatisfactory.
One preferred embodiment of the present invention relates to the nitriding or nitridation of porous silica preforms and involves new technology which appears to be a giant step forward and a breakthrough of potentially great importance in the field of nitrogen-containing silica or silicon oxynitrides. Incredible improvement in the physical properties of a high-purity quartz glass can be obtained by incorporating a minute amount of chemically-bonded nitrogen in the silica.
That first embodiment is remarkable not only because of the difficulty in forming substantial amounts of chemically-bound nitrogen but also because of the difficulty in measuring or detecting the amounts being formed or in ascertaining any benefits therefrom. The improvement obtained in the resistance of quartz glass to devitrification was quite unexpected.
In describing said first embodiment, said copending application Ser. No. 08/269,002 abandoned sets forth different methods for nitridation of silica preforms as described hereinafter. One favored method can be carried out in a single induction furnace wherein the silica preform is heated in nitrogen from a temperature below 1400xc2x0 C. to a temperature preferably above 1700xc2x0 C. Another method comprises a two-stag process wherein the silica preform is first nitrided at a lower temperature, such as 1000xc2x0 C. to 1200xc2x0 C., and is later sintered to high density.
A second preferred embodiment of the invention relates to the use of special refractory dopants or additives to alter and modify the molecular structure and improve the physical properties of sintered quartz glass. Nitrogen-modified and carbon-modified silica glass made according to the invention can be provided with improved resistance to devitrification and increased high-temperature viscosity or sag resistance. The preferred refractory dopants are carbides and nitrides of silicon, such as silicon carbide, silicon nitride or silicon oxynitride. Silicon nitride is a preferred dopant, but nitrides of calcium, aluminum, chromium, titanium and other metals may also be advantageous.
A suitable refractory composition for making nitrided quartz glass articles could, for example, consist essentially of micronized particles of vitreous silica and a small amount (e.g., 0.01 to 0.3 percent by weight) of submicron particles of silicon nitride or other metal nitride that can release nitrogen for reaction with silica during the sintering operation.
Nitridation of the quartz glass in an induction furnace can be effected as the silica preform is heated from 1400xc2x0 C. to 1700xc2x0 C. or above in helium or in a vacuum because of the nitrogen released by the metal nitride. However, it is usually desirable to sinter the silica preform in nitrogen or a predetermined mixture of nitrogen and helium.
In the practice of the present invention, it may be desirable to employ a gel-casting process in making the porous silica preform. Such a process based on ethyl silicate (TEOS), is suitable for production of high-purity quartz glass and is disclosed in some detail in parent applications Ser. Nos. 08/269,002 now U.S. Pat. No. 07/767,691; U.S. Pat. No. 5,389,582 and 08/804,234 U.S. Pat. No. 6,012,304. That disclosure is incorporated herein by reference and made a part of the present continuation-in-part patent application.
A third preferred embodiment of the present invention involves the impregnation of the porous silica preform formed by slip casting, isostatic pressing, electrophoretic deposition or gel casting, for example. After drying and firing, the preform is soaked in or thoroughly impregnated with a suitable silica sol (e.g., a hydrolyzed silicon compound, such as TEOS) as described in said copending applications Ser. Nos. 269,002 and 07/767,691 U.S. Pat. No. 5,389,582. It is then gelled, dried and fired before a subsequent nitriding treatment or a final sintering in an induction furnace.
It has been discovered that such impregnation of the porous preform with a hydrolyzed silicon alkoxide (e.g., ethyl silicate) provides remarkable advantages in the commercial manufacture of transparent quartz glass receptacles, such as crucibles, bell jars and acid tanks. For some reason such treatment of the preform improves the purity of the product by helping to remove sodium ions and other metal ions. It also facilitates the sintering operation and makes it possible to minimize the formation of gas bubbles in the glass during the final helium sintering operation.
The larger pores of a slip-cast silica preform are more apt to cause significant gas bubbles in the sintered glass. These large pores soak up the hydrolyzed alkyl silicate and permit gelling thereof inside the pores. The result of the alkyl silicate impregnation seems to be smaller and more uniform pore size, better suited to the production of transparent quartz glass which has a minimal bubble content.
The present invention provides a process for producing quartz glass of extremely high quality. The sintering of the TEOS-impregnated silica preform can be carried out in two stages in two different furnaces over a substantial period of time rather than in a single induction furnace in 6 to 12 minutes or so. In the first stage, the preform is sintered for a substantial period of time, such as 30 minutes to 3 hours or more, at a high temperature, such as 1400xc2x0 C. to 1500xc2x0 C., to cause a substantial increase in the density of the preform and/or to seal the pores thereof. Unlike the rapid sintering provided in an induction furnace, according to said U.S. Pat. No. 4,072,489, which is non-uniform and intended to provide a temperature gradient, the first stage of the sintering causes relatively uniform heating of the silica and promotes formation or cells with a more uniform pore size.
After a first-stage sintering to a higher density, such as 90 to 95 volume percent, the densified glass article can be placed in the conventional induction furnace and sintered to full density in an inert atmosphere. The temperature can be raised to about 1750xc2x0 C. to melt and eliminate unwanted cristobalite.
In the first stage, the sintering can be carried out in a vacuum at a suitable low pressure (preferably below 5 torrs) or in an atmosphere of helium. In the second stage, after the pores have been sealed, argon or other inert gas can be provided in the induction furnace. If the pores contain helium gas only, it will escape to permit formation of substantially bubble-free full-density glass. A two-stage sintering process according to this invention is effective in producing transparent glass and is also useful in the manufacture of white (opaque) nitrided quartz glass (See Example II).
The process of this invention can also be modified to produce nitrided quartz glass that is clear or semitransparent by employing hot isostatic pressing to compress or remove nitrogen-containing gas bubbles.
In carrying out the invention of said first-named embodiment, a shaped silica body or preform with a porosity of 10 to 40 volume percent can be formed from a refractory silica composition or a slurry of fine silica particles by slip casting, gel casting, electrophoretic deposition, isostatic pressing, injection molding or other suitable method (see U.S. Pat. Nos. 3,222,435 and 3,619,440). The porous silica preform can be formed and treated in such a manner that, after drying and firing, it contains a substantial amount of chemically-bound hydroxyl groups and/or other suitable reaction-promoting groups or ions (e.g., at or near the inner surfaces of the voids or pores). These groups or ions are usually hydroxyl groups or calcium ions. The fired porous silica preform is then nitrided in an appropriate manner (e.g., in an atmosphere of nitrogen in an induction furnace heated to 1700xc2x0 C. or higher or in an atmosphere of anhydrous ammonia maintained at a suitable high temperature). To assure that the pores of the preform are filled, a substantial vacuum can be employed to remove air or other gas from the pores of the preform before nitrogen gas or other nitrogen-containing gas is introduced to those pores. Also a pressure differential can be provided to force the nitrogen gas through the porous preform.
The final sintering of the preform to a high density, such as 98 to 99 weight percent, can be carried out in an electric induction furnace generally as disclosed in U.S. Pat. No. 4,072,489 using a nitrogen atmosphere rather than an atmosphere of helium. The glass is usually heated to at least 1700xc2x0 C. during sintering and is preferably heated to about 1750xc2x0 C. or above the melting point of cristobalite to eliminate crystalline silica.
Optionally, the porous silica preform can be treated prior to nitridation to obtain improved results. The treatment can include impregnation with a hydrolyzed silicon alkoxide as described in copending applications Ser. Nos. 07/767,691 U.S. Pat. No. 5,389,582 and 08/269,002, abandoned and can include a hydroxylation treatment to increase the number of hydroxyl groups. In one embodiment of the invention, the porous preform is heated in a furnace atmosphere of air or oxygen and steam to a high temperature, such as 400xc2x0 C. or higher to increase the hydroxyl content of the glass prior to the the nitriding step.
Parent application Ser. No. 08/269,002 abandoned describes a preferred embodiment of the invention wherein a preform comprising a porous silica body is made by shaping a refractory composition comprising essentially fine particles of high-purity silica and the shaped preform is then fired at at temperature of at least 1000xc2x0 C. Thereafter the fired silica preform is heated in a furnace in a nitrogen atmosphere to a sintering temperature above 1600xc2x0 C. while the pores are filled with nitrogen gas.
This process of nitridation is characterized in that the fired silica preform preferably contains chemically-bound hydroxyl and/or calcium ions that promote nitriding and provide a multiplicity of closely spaced accessible reduction sites scattered throughout the preform and located at or near internal pore surfaces of the preform contacted by the nitrogen. During the final sintering operation, normally carried out in an induction furnace, the preform is heated to a temperature of 1700xc2x0 C. to 1750xc2x0 C. or higher in such manner as to cause reaction at said reduction sites and to provide the quartz glass with an effective amount of chemically-combined nitrogen no less than 25 parts per million (ppm) and preferably 40 to 50 ppm or more.
This type of process is attractive for commercial manufacture because of its simple and economical nature. The preform is preferably made from an aqueous slurry or slip and is preferably formed by slip casting. In order to provide calcium ions to promote nitridation of the silica, the slip casting molds are preferably formed of gypsum or hydrated calcium sulfate and the slips used for casting can contain small amounts of calcium, preferably at least 2 or 3 ppm and no more than 20 ppm.
The present invention involves a remarkable discovery that nitridation of a silica preform can be effected in a similar way in an induction furnace by an alternative process wherein the preform is heated to 1700xc2x0 C. or above in an atmosphere of helium or argon, rather than in a nitrogen atmosphere. In the alternative process the high-purity silica slurry or slip used to make the preform preferably contains micronized particles of silicon nitride or other suitable metal nitride that are uniformly distributed throughout the slip, preferably by milling the slip for 24 to 30 hours or more. Such particles preferably have an average particle size below one micron when manufacturing dense quartz glass products with a density greater than 90 percent.
The alternative process using a refractory dopant, such as silicon nitride or silicon carbide has special advantages when used to make quartz glass products of low density because of the unique ability to liberate gas as the silica approaches or reaches the melting point and to form a closed-cell structure with bubbles of substantial size.
Silicon nitride and silicon carbide particle additions are also advantageous when making quartz glass products with a higher density. The use of such refractory dopants may be a major breakthrough in the field of quartz glass and a giant step forward because of its potential for producing various types of engineered glass products with special utility. It appears that silicon nitride is particularly well suited for nitridation of silica because of its ability to release or produce nascent nitrogen at a time when the chemical bonds between the silicon and oxygen atoms of the silica are weakened by heat and vulnerable to reaction.
In the practice of this invention, it is usually desirable to employ silicon nitride particles having an average particle size of from about 0.2 to about 2 microns, but it is sometimes preferable to employ smaller particles with an average particle size of 0.1 micron or less to produce quartz glass with gas bubbles of small size. The silicon nitride particles can be of submicron size or colloidal size.
Other methods may be employed to provide quartz glass with a large number of small bubbles in a somewhat similar way. In another embodiment of this invention, small particles of an organic or combustible carbon compound are thoroughly mixed and dispersed in the silica slip or slurry before molding or casting the silica preform. The organic material should be compatible in the process and capable of being uniformly dispersed to provide a suitable suspension or dispersion. Various carbon compounds or binders can be used including polystyrene, starch, vinyl resins, microcrystalline cellulose, and the like.
Other embodiments of the invention relate to improvements in tube furnaces and other high-temperature equipment employed in the semiconductor industry which incorporate quartz glass components made in accordance with the invention. The processing operation in a tube furnace is more economical and efficient and can be greatly improved when the quartz furnace tube is provided with a cup-shaped end cap formed of special opaque quartz glass made according to the present invention. Such an end cap can provide a remarkably effective radiation heat shield in apparatus of the type illustrated in FIG. 1 of the drawings. Radiation heat transfer can be further reduced when portions of the quartz tube are also made of special quartz glass as illustrated in FIG. 1.
A number of unique and remarkable products can be produced when practicing the present invention. Special quartz glass products made according to the invention exhibit remarkable physical properties and can be of great commercial value. The nitridation of a porous vitreous silica preform in accordance with the invention apparently causes nitrogen atoms to become chemically bonded to silicon atoms of the vitreous silica, thereby effecting a remarkable change in the physical properties of the quartz glass even when the nitrogen content is barely measurable (e.g., below 0.005 percent by weight).
The resistance of the quartz glass to devitrification at high temperatures (e.g., 1100xc2x0 C. to 1300xc2x0 C. or higher) can be drastically improved by nitridation, perhaps more than fifty-fold and possibly two orders of magnitude. At the same time the high-temperature viscosity or resistance of the glass to deformation at high temperatures, such as 1250xc2x0 C. or higher, can be increased dramatically (e.g., 50 to 100 percent or more). Because of their remarkable properties, special quartz glass products made according to the invention are valuable for a wide variety of uses in the chemical and electronic arts and other scientific arts and include hell jars, crucibles, furnace tubes, tanks, trays, and plates and tiles for furnaces, reactors and hot-wall applications. Such products are particularly useful in the semiconductor industry and the field of microelectronics because of extreme purity, uniformity and reliability.
The opaque nitrided quartz glass of this invention is particularly valuable for radiation heat shields used in CVD furnaces for chemical vapor deposition and in epitaxy reactors, diffusion furnaces and other furnaces used in the semi-conductor industry. Such glass is extremely well suited for such uses.
Clear or semitransparent quartz glass which has been nitrided during sintering according to the present invention and then pressurized by hot isostatic pressing or in a hipping furnace has great potential because of improved resistance to sag and devitrification. Such nitrided quartz glass can be used, for example, to make furnace tubes with increased useful life.